Pseudorabies virus (PRV) gene

ABSTRACT

The present invention provides a recombinant DNA sequence encoding the pseudorabies virus (PRV) glycoprotein gX, host cells transformed by said DNA sequence, the gX polypeptide, and methods and kits for detecting animals infected with PRV.

This application is a continuation of patent application Ser. No. 158,980, filed Feb. 22, 1988, now abandoned, which is a division of copending patent application Ser. No. 853,087, filed Apr. 17, 1986, which is a continuation-in-part of patent application Ser. Nos. 606,103, filed May 2, 1984, and 606,307, filed May 2, 1984, both now abandoned.

BACKGROUND OF THE INVENTION

Pseudorabies virus (PRV) is a herpesvirus which can infect most domestic animals to cause clinical symptoms ranging from vomiting, diarrhea, fever, respiratory problems, neurological disorders, abortion, and culminating, generally, in death to the host animal.

Pseudorabies is known as Aujeszky's disease, mad itch and bulbar paralysis. When PRV infects pigs, a serious economic loss is incurred by the farmer. Very young pigs die whereas older pigs get sick and lose weight. Pregnant sows will abort when infected with PRV. Thus, an effective program for the eradication of PRV in swine is urgently needed.

Herpesviruses are among the most complex of animal viruses, coding for the synthesis of at least 50 virus specific proteins. Among the most immunologically reactive proteins of herpesviruses are the glycoproteins found in virion membranes and in the membranes of infected cells. The literature on pseudorabies glycoproteins (Ben-Porat, T., and Kaplan, A. S. Virology 41, 265-273, 1970; Kaplan, A. S., and Ben-Porat, T., Proc. Natl. Acad. Sci. USA, 66, 799-806, 1970) indicates that there are at least four viral glycoproteins in infected cells and virons.

Glycoproteins excreted into the medium have been reported for several herpesviruses, including herpes simplex virus (Kaplan, A. S., Erickson, J. S., and Ben-Porat, T., Prog. Med. Virol. 21, 1-12, 1975); herpes saimiri virus (Randall, R. E., and Honess, R. W., J. Gen. Virol. 51, 445-449, 1980); Marek's disease virus (Van Zaane, D., Brinkhof, J. M., Westenbrink, F., and Gielkens, A. L., Virology 121, 116-132, 1981); and pseudorabies virus (Erickson, J. S., and Kaplan, A. S., Virology 55, 94-102, 1973). In each of these cases except PRV, the excreted glycoprotein(s) is reported to be a subset of the virion glycoproteins. The publication by Ben-Porat and Kaplan, 1970, discloses that the PRV excreted glycoprotein is distinct from virion glycoproteins. This glycoprotein is known as 3a in the prior art (see Erickson and Kaplan, 1973, supra) and is referred to hereinafter as glycoprotein gX. Glycoprotein gX has the following characteristics when harvested from PRV-infected cells:

(1) it is the predominant protein in the culture medium of PRV infected animal cells in culture;

(2) it is a glycoprotein;

(3) it has a molecular weight of about 95 kilodaltons by standard SDS polyacrylamide gel electrophoresis (DATD cross-linked gel);

(4) it is a sulfated protein;

(5) it is soluble in about 1% perchloric acid;

(6) it is immunogenic in standard laboratory mice; and,

(7) it raises an immune response in the animal host which response is protective against a lethal challenge by a virulent strain of PRV.

We have isolated and mapped from the unique small region of pseudorabies virus (PRV) genome the gene encoding the PRV excreted glycoprotein gX and cloned said gene to express a fragment of the protein.

DESCRIPTION OF FIGURES AND CHARTS

FIG. 1 depicts the BamHI restriction sites of the PRV genome with an enlargement of the region wherein the gX gene is located. The gX gene region is identified by the dark bar line (5'-3') with relevant restriction sites identified.

FIG. 2 depicts the structure of pUC1129 and details of the region wherein the gX gene is located.

FIG. 3 depicts the construction of the plasmid p60-11.

FIG. 4 depicts the construction of plasmid pTRZ1.

FIG. 5 depicts the construction of plasmid pTRZ4.

Charts 1A, 1B and 1C depict the base sequence of the gX gene.

Charts 2A, 2B and 2C depict the amino acid sequence of the gX protein.

SUMMARY OF INVENTION

The present invention provides a host cell transformed with a recombinant DNA molecule or a sequence of DNA basepairs comprising a DNA sequence of DNA basepairs encoding the PRV glycoprotein gX and fragments thereof and functional equivalents thereof.

More particularly the present invention provides a host cell transformed with a recombinant DNA molecule or a sequence of DNA basepairs comprising a DNA sequence of the formula set forth in Charts 1A, 1B and 1C hereof and fragments thereof and functional equivalents of said molecule and said fragments.

The present invention also provides a polypeptide expressed by a host transformed with a recombinant DNA molecule comprising a DNA sequence of the formula set forth in Charts 1A, 1B and 1C hereof and immunogenic fragments thereof and functional equivalents of said polypeptide and said immunogenic fragments.

More particularly the present invention provides a polypeptide having the formula set forth in Chart 2 and immunogenic fragments thereof and functional equivalents thereof.

The present invention also provides a recombinant sequence of DNA basepairs comprising DNA basepairs encoding the pseudorabies virus glycoprotein gX or immunogenic fragments thereof.

DETAILED DESCRIPTION OF INVENTION

The location of the gene encoding the major excreted glycoprotein of PRV, i.e., gX, was mapped using hybrid selection/translation experiments and restriction endonuclease analysis of subclones of PRV DNA fragments.

We cloned XH01 restriction fragments of PRV DNA (PRV rice strain) into a plasmid that contains a unique XH01 site to give the plasmid pUC1129. Hybridization of cytoplasmic RNA from PRV infected cells followed by translation in vitro to determine that the hybridized m-RNA coded for the gX protein was performed. pUC1129 is available from Escherichia coli HB101, NRRL B-15772. This biologically pure culture is available from the permanent collection of the Northern Regional Research Center Fermentation Laboratory (NRRL), U.S. Department of Agriculture, in Peoria, Ill., U.S.A.

E. coli HB101 containing pUC1129 can be grown up in L-broth by well known procedures. Typically the culture is grown to an optical density of 0.6 after which chloramphenicol is added and the culture is left to shake overnight. The culture is then lysed by, e.g., using high salt SDS and the supernatant is subjected to a cesium bromide/ethidium bromide equilibrium density gradient centrifugation to give the plasmids.

pUC1129 (FIG. 2) provides a source for the gene encoding the PRV glycoprotein gX. The availability of the gene sequence permits direct manipulation of the gene structure which concomitantly affects regulation of expression and/or the structure of the protein encoded by the gene. Now that the gX gene map is known, it is possible to clone the gene or portions thereof from any strain of PRV using this sequence as a hybridization probe, and express the entire gX protein or fragments thereof by recombinant techniques generally known in the art. Knowledge of the gX gene sequence enables us to deduce the amino acid sequence of the gX protein, and hence, the polypeptide may be produced by chemical synthesis as well as recombinant DNA techniques.

The primary structure (sequence) of the gene coding for gX and flanking regions of the DNA (5' and 3') is set forth in Charts 1A, 1B and 1C. The gene contains regulatory elements similar to those flanking most eucaryotic genes (e.g., promoter region, TATA box, appropriate initiation and termination codons marginating a large open reading frame, and polyadenylation signal(s)) and inferred from the sequence of the structural region of the gene structures like those expected in an excreted protein (e.g., signal sequence). The gene is relatively GC rich as is the entire PRV genome. The codon utilization of the gene reflects this abundance of G and C nucleotides. There are several interesting symmetries observed in the DNA sequence of which the most remarkable is a direct repeat of 50 basepairs with 100% homology. The repeat appears about 50% longer if one considers conservation of amino acid residues. An additional unusual feature is the presence of several "homopolyamino" acid segments in the protein (11-13 residues in length). The total length of the region of the PRV genome we have sequenced is greater than 2000 basepairs. The length of the open reading frame (from ATG to UGA) is 1497 nucleotides (>71% GC).

The gX gene or fragments thereof can be extracted from pUC1129 by digesting the plasmid DNA from a culture of NRRL B-15772 with appropriate endonuclease restriction enzymes, most suitably enzymes which will cleave at the sites of the gene depicted in FIG. 1. The restriction endonucleases are commercially available and their use is well known in the art and usually is provided by the commercial supplier of the enzyme. For example, as described in more detail hereinbelow, the fragment BamHI/PvuII was isolated by digestion of a preparation of pUC1129 with BamHI and PvuII, and isolation by gel electrophoresis.

The excised gene segment(s) can be ligated to various cloning vehicles or vectors for use in transforming a host cell. The vector may be a replicating vector, i.e., a vector which replicates within the host cell, or an integrating vector, i.e., a vector which serves to transport the exogenous gene material into the host cell and integrate said gene material with the DNA of the host cell. The vector contains heterologous gene sequences to initiate transcription and translation of the gX gene which are compatible with the host cell to be transformed. When the host cell is a higher animal cell, e.g., a mammalian cell, the naturally occurring transcription and translation gene sequence of the gX can be employed or the naturally occurring translation gene sequence can be employed along with a heterologous promoter sequence. The vectors additionally contain a marker to provide a phenotypic trait used in selection of the transformed host cells. Additionally a replicating vector will contain a replicon. Typical vectors are plasmids and phages. In essence, one can use any DNA sequence that is capable of transforming a host cell such that said host cell is rendered capable of expressing the gX protein or portions thereof. Thus the present invention provides as a novel entity a sequence of DNA comprising basepairs encoding the PRV glycoprotein gX. By basepairs is meant the textbook definition of adenine (A) and thymine (T), and cytosine (C) and guanine (G) as occurs in DNA sequences.

The term host cell as used herein means a cell transformed with the gX gene and thus rendered capable of expressing the gX protein. The host cell can be procaryotic or eucaryotic. Illustrative procaryotic cells are bacterial microorganisms such as E. coli, B. subtilis, Pseudomonas, and B. stearothermophilus. Illustrative of eucaryotic cells are yeast, such as Saccharomyces cerevisiae (baker's yeast) or higher animal cells such as cells of insect or mammalian origin. Mammalian cell systems often will be in the form of monolayers of cells although mammalian cell suspensions may also be used. Illustrative examples of mammalian cell lines include VERO and HeLa cells, Chinese hamster ovary (CHO) cell lines, WI38, BHK, COS-7 or MDCK cell lines.

As indicated above the vector, e.g., a plasmid, which is used to transform the host cell contains a gene sequence to initiate the transcription and translation of the gX gene sequence. When the host cells are bacterial microorganisms illustrative useful transcription and translation gene sequences are obtained from the trp promoter (Goeddel, et al., Nucleic Acids Res. 8, 4057 (1980)); the lac promoter (Chang, et al., Nature 275, 615 (1978)); the outer membrane protein promoter (EMBO J. 1, 771-775 (1982)); the bacteriophage λ promoter (Nucleic Acids Research 11, 4677-4688 (1983)) or the α-amylase (B. subtilis) promoter. When the host cell is yeast illustrative useful transcription and translation gene sequences are obtained from, e.g., α-mating factor. When the host cell is of insect or mammalian origin illustrative useful transcription and translation gene sequences are obtained from the SV-40 promoter (Science 222, 524-527 (1983)) or e.g., the metallothionein promoter (Nature 296, 39-42 (1982)). As noted above, when the host cell is mammalian, one may use the transcription and translation gene sequence for the gX gene but preferably in combination with a heterologous transcription initiation site. The plasmid or replicating or integrating DNA material containing the transcription or translation gene sequence is cleaved using restriction enzymes and adjusted in size as necessary or as desirable and ligated with the gX gene or fragments thereof by means well known in the art. When yeast or higher animal host cells are employed, terminator sequences from known yeast or mammalian genes are incorporated into the vector. Additionally, gene sequences to control replication of the host cell may be incorporated into the vector.

The host cells are competent or rendered competent for transformation by various means. When bacterial cells are the host cell they can be rendered competent by treatment with salts, typically a calcium salt, as generally described by Cohen, PNAS 69, 2110 (1972). A yeast host cell generally is rendered competent by removal of its cell wall or by other means such as ionic treatment (J. Bacteriol. 153, 163-168 (1983)). There are several well-known methods of introducing DNA into animal cells. These include: calcium phosphate precipitation, fusion of the recipient cells with bacterial protoplasts containing the DNA, treatment of the recipient cells with liposomes containing the DNA, and microinjection of the DNA directly into the cells.

The transformed cells are grown up by means well known in the art (Molecular Cloning, Maniatis, T., et al., Cold Spring Harbor Laboratory, 1982; Biochemical Methods in Cell Culture and Virology by Kuchler, R. J., Dowden, Hutchinson and Ross, Inc., 1977; Methods in Yeast Genetics by Sherman, F., et al., Cold Spring Harbor Laboratory, 1982) and the expressed gX protein is harvested from the cell medium in those systems where the protein is excreted from the host cell or from the cell suspension after disruption of the host cell system by, e.g., mechanical or enzymatic means which are well known in the art.

As described in detail in the specific Examples, the identity of the expressed gX protein was confirmed by immunoprecipitating labeled gX protein sera from mice injected with the protein.

As noted above the amino acid sequence of the gX protein as deduced from the gX gene structure is set forth in Chart 2. Immunological fragments of the gX protein are any portions of the polypeptide sequence which are capable of eliciting an immune response in an animal such as a mammal which has been injected with the polypeptide sequence. Illustrative examples of portions of the gX protein which are immunological fragments thereof are the following which are represented along with the DNA fragment coding for the polypeptide. In the codon the letter N represents adenine, thymine, cytosine or guanine; Q represents cytosine or thymine; and P represents guanine or adenine. ##STR1##

As indicated hereinabove the entire gene coding for the gX protein can be employed in constructing the vectors and transforming the host cells to express the gX protein or fragments of the gene coding for the gX protein can be employed whereby the resulting host cell will express portions or fragments of the gX protein. Examples of fragments of the gX gene which can be employed are set forth above with the polypeptide encoded thereby. Any fragment of the gX gene can be employed which results in the expression of a polypeptide which is an immunogenic fragment of the gX protein.

The following examples illustrates the present invention whereby the host cell, E. coli is transformed with a plasmid p60-11; however, it will be readily apparent to one skilled in the art that a variety of vectors could be constructed which would be suitable for transforming a host cell rendering it capable of expressing the gX protein. The recombinant DNA methods employed in practicing the present invention are standard procedures described in detail in Molecular Cloning, T. Maniatis, et al., Cold Spring Harbor Laboratory, 1982. Other references detailing techniques applicable in practicing the present invention include European patent application nos. 093,619 and 032,134.

EXAMPLE 1

(a) Construction of Plasmid p60-11 (see FIG. 3)

The plasmid pTRZ4 contains a strong promoter, the trp promotor, and the trpLE translation initiation site with 12 amino acids from trpLE. The vector also contains the lacZ gene, minus its translation initiation site, immediately downstream from but out of phase with the trpLE sequences. A unique SmaI restriction site between the trpLE and lacZ sequences allows insertion of DNA fragments. Expression of the lacZ sequences requires insertion of a fragment containing no termination codons in a phase that joins the trpLE and lacZ sequences in phase.

A 1.5 kilobase BamHI/PvuII fragment containing a portion of the gX gene was isolated from pUC1129. A random population of subfragments of the BamHI/PvuII fragment was generated by a varied time of digestion of the fragment with Bal31 nuclease. This entire population of random fragments was ligated into the SmaI site of pTRZ4, which is between the trpLE and lacZ sequences, and a lac minus E. coli was transformed with the ligation mixture. The transformants were plated on Xgal plates to screen for β-galactosidase expression. One of the darkest blue colonies was isolated to obtain the modified pTRZ4 plasmid which we named p60-11. The p60-11 plasmid was found to contain a BamHI/EcoRI fragment of approximately 900 basepairs, indicating an insertion of approximately 600 basepairs in the BamHI/EcoRI fragment of pTRZ4.

(b) Protein Synthesis Directed by p60-11

The overproduced protein from p60-11 was found to be in an aggregated, insoluble form inside the cells, and this property could be used to purify the protein. The E. coli strain carrying plasmid p60-11 was grown to stationary phase in L broth, and diluted 1:100 in M9+1% casamino acids to induce synthesis of the recombinant protein. When the optical density of the culture exceeded 1.0, the cells were harvested and resuspended in 50 mM Tris HCl, pH 7.6, 0.3M NaCl, 0.5 mM EDTA (5-10 ml/liter of culture). Egg white lysozyme (Sigma) was added to a concentration of 1 mg/ml, and the reaction incubated on ice for 15 minutes. Then NP40 was added to a final concentration of 0.2%, and incubation continued for an additional 15 minutes on ice. The extract was sonicated until no longer viscous, and then centrifuged at 10,000 rpm for 15 minutes in a Sorvall HB-4 rotor. The recombinant p60-11 protein was found almost entirely in the pellet.

When the protein in these cells was analyzed using a Coomassie blue-stained SDS electrophoresis gel, the most abundant protein in the cells was seen to be a protein of apparent molecular weight of approximately 170,000. This is consistent with a hybrid protein that would contain an N-terminal trpLE sequence, a PRV glycoprotein coding sequence, and most of the lacZ at the C-terminus.

(c) Purification of p60-11 Protein

The p60-11 protein was purified by electrophoresis (SDS/PAGE) using 9.25% polyacrylamide gels with DATD as generally described by Morse, L., et al., J. Virol. 26, 389-410, 1978. Preparative gels for isolation of the protein were 3.0 mm in thickness and contained a 120 mm lane along with a 7 mm marker lane. Two ml of a boiled p60-11 pellet in 10 ml SDS sample buffer (4% SDS, 10% 2-mercaptoethanol, 6.0% sucrose, 200 μg/ml bromphenol blue, 100 mM Tris-HCl, pH 7.0) was applied to the wide lane and electrophoresed. A vertical slice was made in the gel to contain both the marker lane and 10 mm of the preparative lane; this was placed into Coomassie stain for 1 hour, and the remainder of the gel was covered with plastic wrap and placed at 4° C. After destaining the strip for approximately 1 hour (or until it's size matched perfectly with the unstained gel), the appropriate 170 kilodalton band was sliced out by using the stained molecular weight standards and prep lane bands as a guide. The gel slice was chopped into small pieces and transferred into a 10 ml syringe. This syringe was connected by tubing to another syringe. Five ml of gel elution buffer (0.05% SDS/0.05M NH4 HCO3) was then added, and was carefully forced through each syringe to macerate the acrylamide as much as possible. The contents were finally placed into a capped 50 ml polypropylene tube, 5 ml of gel elution buffer added, and the parafilm-sealed tube was shaken for 6 hours at 37° C. To remove unwanted acrylamide, the material was applied to a 30 ml syringe stoppered with glass wool. The eluted protein was collected by precipitation with 9 volumes of acetone, resuspended in 10 mM Tris-saline (pH 7.0), reverified via SDS/PAGE analysis, and stored at -20° C.

(d) Animals and Immunizations

Female ICR mice (age 2-4 months) were injected subcutaneously and intraperitoneally with macerated acrylamide containing 50 μg p60-11 protein mixed 1:1 with complete Freund's adjuvant. After three weeks' rest, 60-90 μg of acetone precipitated, solubilized fusion protein mixed 1:1 with incomplete Freund's adjuvant was injected intraperitoneally. This was repeated every 10 days over six weeks' time. Serum samples were collected prior to each of the last two intraperitoneal boosts.

(e) Immunoprecipitations

The IgG binding affinity of S. aureus Cowan I cells (Calbiochem or Bethesda Research Laboratories) was utilized to precipitate immune complexes. Staph cells were prewashed in Buffer A (150 mM NaCl, 5 mM EDTA, 0.1% Triton X-100, 0.5% NonidetP-40, 50 mM Tris-HCl, pH 7.4), Buffer B (same except 0.05% Nonidet P-40), and resuspended in Buffer C (Buffer B plus 1 mg/ml ovalbumin). ¹⁴ C-glucosamine-labeled infected cell polypeptides (ICP's) or ¹⁴ C gX (i.e., the medium from ¹⁴ C-glucosamine-labeled cells) served as antigen (see description below under Cells and Virus) and was preincubated 1:1 with washed staph cells at 4° C. The prepared antigen supernatant was carefully removed after ultracentrifugation at 35,000 rpm for 45 minutes in a SW 50.1 rotor (Beckman). Twenty μl of antiserum was then incubated with 125 μl of antigen for one hour at 4° C., followed by the addition of 200 μl washed staph and a similar incubation step. The staph-antibody-antigen mixtures were then microfuged and washed 3 times in RIPA/SDS buffer (150 mM NaCl, 1% Triton X-100, 0.1% SDS, 1 mg/ml ovalbumin, 10 mM Tris-HCl, pH 7.2) and the supernatant drained well. The final pellet was resuspended in 50 μl of 2X SDS sample buffer and boiled for one minute. Staph cells were then pelleted and the supernatants applied to a 9.25% polyacrylamide gel for electrophoresis. The gels were made 0.75 mm thick by the procedure of Morse, L., et al., supra, and the gels were run for 16 hours at 5 mA/mm thickness. Anti-p60-11 serum pool, non-immune mouse serum, and immune pig serum (from a PRV-recovered animal) were independently assayed for immunoprecipitation of ¹⁴ C gX and ¹⁴ C ICP antigens. Anti-p60-11 exclusively precipitates the gX antigen as compared to the controls. A similar band is immunoprecipitated by this serum in the ¹⁴ C ICP experiment, along with a faintly immunoprecipitated glycoprotein of unknown identity at 61 kilodaltons. Both bands are precipitated by the immune pig serum.

(f) Cells and Virus

Cells used for virus propagation and protein production were African Green Monkey (C. aethiops) kidney cells (ATCC CCL81;=VERO). Cells were maintained in Dulbecco's modified Eagle's medium (GIBCO) supplemented with 100 units/ml penicillin, 100)g/ml streptomycin, 10 mM HEPES (pH 7.4), 2 mM glutamine, and 10% fetal bovine serum (KC Biological). Primary rabbit kidney cells were grown in the same medium.

Pseudorabies virus was isolated from a natural outbreak of PRV in swine. Virus stocks were prepared by infecting either Vero or rabbit kidney cells at low multiplicity (F 0.01). Although no properties of PRV are known to be dependent upon the cell line in which the virus is grown, we note that the biochemical experiments were done with virus grown on Vero cells, and animal challenge experiments were done with virus grown on rabbit kidney cells.

Cells were infected with between 5 and 10 plaque forming units per cell in medium 199 (GIBCO) supplemented with 1% fetal bovine serum. At 4 hours post infection, ¹⁴ C glucosamine was added (10 μC in 2.5 ml medium per 25 cm² flask of cells; New England Nuclear, 54.2 mC/mmole). At 12-16 hours post infection, the medium was removed from the culture and stored frozen at -70°.

ICP's were harvested by shaking the infected cells into the medium and subsequent centrifugation. The cells were washed in phosphate buffered saline (PBS) without magnesium or calcium, and then resuspended in PBS minus magnesium and calcium, plus 1% NP40, 0.5% sodium deoxycholate, 0.1% SDS, and 200 μg/ml ovalbumin. Debris was removed from the lysed cells by centrifugation and the labeled ICP's were stored at -80° C.

ICP's were labeled with ³⁵ S-methionine by the same procedure, except the label added at 4 hours post infection was ³⁵ -methionine (1.5 mCi in 7.5 ml medium per 75 cm² flask of cells) in 199 medium with 1/10 normal concentration of methionine.

The pTRZ4 plasmid was constructed as depicted in FIGS. 4 and 5. FIG. 4 shows the construction of pTRZ1 which contains the E. coli trp promoter and translational initiation region of the leader peptide and the E. coli lacZ gene missing a few N-terminal amino acids, which would still have β-galactosidase activity. These two elements are joined in the vector such that the lacZ sequences are out of phase with respect to the translation frame of the trp gene.

After insertion of DNA fragments between the trp and lacZ sequences of such a vector, screening for expression of the β-galactosidase activity will then allow identification of plasmids expressing a hybrid protein.

The lacZ gene minus the N-terminal eight amino acids was obtained from pMC1403 (M. J. Casadaban, et al., J. Bacteriol. 143, 971-980 (1980). This plasmid has three unique restriction enzyme cleavage sites. pMC1403 was cut with EcoRI, dephosphorylated with BAP, and the ends made blunt with T4 DNA polymerase. The plasmid used as a source of the trp promoter was pVV1 (B. P. Nichols and C. Yanofsky, Methods in Enzymology, eds. L. Grossman and K. Moldane, 101, 155-164, Academic Press, N.Y., 1983). pVV1 contains the trp operon, with a 952 basepair deletion (ΔLE1413) fusing the trp leader peptide to the trpLE gene. A 323 basepair fragment was isolated by digestion of pVV1 with PvuII plus BglII and and preparative agarose gel electrophoresis (FIG. 4). The BglII site was filled by T4 DNA polymerase. The fragment was then ligated into the vector to give pTRZ1.

The use of pTRZ1 in the construction of pTRZ4 is shown in FIG. 5.

The plasmid pMC1871 (S. K. Shapira, et al., Gene 25, 71-82 (1983)) was used as a source of the C-terminal end of the lacZ gene without the lacY gene. This new C-terminus of lacZ was joined at the SacI site of the lacZ gene of pTRZ1 deleting all lac sequences downstream from the SacI site of pTRZ1. The resulting plasmid, pTRZ4, has the out-of-phase trpLE-lacZ fusion from TRZ1, with no lacY or lacA sequences.

EXAMPLE 2

In this example we set forth the use of the gX (p60-11) polypeptide in a diagnostic test to detect animals infected with PRV. While we used an ELISA assay (A. Voller, et al., The Enzyme-Linked Immunosorbent Assay (ELISA). Dynatech Laboratories, London (1979)), other similar assays known to those skilled in the art may also be employed. Radioiodination of p60-11 Protein

The iodinating agent, 1,3,4,6-tetrachloro-3α,6α-diphenylglycouril (TCDG) was coated on the surface of a glass vial by evaporating 200 μl of a chloroform solution containing 1.0 mg TCDG/ml. Iodination proceeded by adding 200 μl of 0.5M sodium phosphate (pH 8.9) to the vial followed by 10 μl of ¹²⁵ I (0.5 mCi). After mixing and a 30 second incubation, 10 μl of the p60-11 protein was added to the vial (p60-11 protein was dissolved at 500 μg/ml in 50 mM borate, 0.9% NaCl, pH 8.8). This mixture was mixed well and incubated for 45 seconds. Then the reaction mixture was chromatographed over Bio-Gel P-6 DG (Sigma Company, St. Louis, MO; 7 mm×185 mm column) using the borate buffer containing 25 μg p60-11 protein/ml. Eight-drop fractions were collected and the ¹²⁵ I in each fraction was determined. The first radioactive fractions were assumed to contain the radioiodinated protein and these were saved for subsequent analysis and use.

ELISA USING THE P60-11 PROTEIN

The antigen solution was prepared by diluting the p60-11 protein to 20 μg/ml in Voller's buffer (50 mM NaHCO₃, 0.03% NaN₃, adjusted to pH 9.6 with NaOH). This was filtered through a 0.45 micron sterilizing filter (Sartorius SM 165 55 K). If needed, the filtered solution was further diluted with Voller's buffer before use.

100 μl of the p60-11 protein in Voller's buffer (concentration about 2 μg/ml) was added to each well of a 96 well plate (Costar 3590 EIA). Adsorption occurred during an overnight, room-temperature incubation. The wells were washed three times with 300 μl of Dulbecco's PBS (8 g/l NaCl, 0.2 g/l KCl, 0.2 g/l KH₂ PO₄, and 1.14 g/l Na₂ HPO₄ ; resultant pH was 7.3-7.4). Unreacted sites on the plastic surface were neutralized during a 1 hour room-temperature incubation with 3% BSA in Dulbecca's PBS (200 μl per well). A single wash of each well with 300 μl of Dulbecco's PBS followed. Then the adsorbed antigen was reacted with antibodies in 100 μl of diluted serum (obtained from pigs exposed to PRV) during a 1 hour room-temperature incubation. Unreacted antibodies were removed by three washes with Dulbecco's PBS (300 Ml/well). Then 100 Ml of a 1/1000 dilution of Protein A-horseradish peroxidase conjugate (diluted in 50 mM Tris, 0.05% Tween-20, 1% BSA, 0.02% NaN₃, pH 8.0) was added to each well for a 1 hour, room-temperature incubation. Again, the wells were washed three times with Dulbecco's PBS (300 Ml/well). One hundred μl of substrate solution was added to each well. This solution was prepared by adding 10 mg of o-phenylenediamine (previously dissolved in 0.5 ml CH₃ OH) and 25 μl of 30% (w/v) H₂ O₂ to 49.5 ml buffer (17 mM Citric acid, 65 mM phosphate and 0.01% merthiolate adjusted to pH 6.3 with NaOH). The enzyme reaction continued for 15 to 35 minutes at room-temperature before 100 μl of 4.5 M H₂ SO₄ was added to each well. Absorbance of the chromophore was measured at 492 nmeters using a Titertek Multiskan. The optimal assay included 1% BSA in the serum dilution buffer and preferably a serum dilution which yields an absorbance of 0.4 or less. With this technique the signal to noise ratio was about 25 and made the assay effective for determining that a serum sample is from an infected pig.

By using this method, it is also possible to distinguish between vaccinated and infected animals. For example, pigs vaccinated with a PRV vaccine virus which does not produce glycoprotein gX (see, e.g., copending U.S. patent application Ser. No. 760,130, filed July 29, 1985) can be distinguished from pigs infected with virulent PRV which do produce gX by serological testing employing the gX polypeptide and immunological fragments thereof. Likewise the polypeptides and methods of the present invention can also be use to distinguish between, for example, pigs vaccinated with a PRV subunit vaccine (see, e.g., copending U.S. patent application Ser. No. 784,787, filed Oct. 4, 1985) and infected pigs, in a similar manner.

The gX polypeptide or immunological fragment or fragments thereof used for these diagnostic purposes can be placed in a mercantile kit comprising multiple containers wherein one of the containers has therein the pseudorabies virus (PRV) glycoprotein gX or an immunological fragment or fragments thereof. ##STR2## 

What is claimed is:
 1. An immunogenic polypeptide pseudorabies virus glycoprotein X (gx) of the formula:and fragments of said polypeptide which retain immunoreactive properties as found in claimed sequence and immunological fragments thereof and functional equivalents of said polypeptide and immunological fragments. 